Thanks to Gorgon, who took a look over my saved .bic and was able to tell me this:

Long version: It is trying to use the "Nightfang Elites Key" in your "Spell Component Pouch" on the first page of inventory. Keys can't normally be quick-slotted, so you it keeps giving that message. You can also drop the key directly from inventory and pick it back up right away, and that will fix it, but dropping the bag with key inside won't (take it out of the bag before dropping).

Now I had emptied all my quick-slots, and removed all gear, and even placed some items I thought may be responsible into a chest, and saved/re-logged. I never thought that an 'empty' looking quick-slot could have something that is not 'quick-slottable' set there! I don't recall if that particular slot previously had a potion or gadget or some emote text there, my understanding was that emptying quick-slots would clear any errors associated with them.

The fix for anyone else having a similar persistent error is: set something in your main inventory to every quick-slot, then re-set the slots as you want them. I knew the troublesome slot, so simply set my bow to that slot and went thru a trans: no error message! You could no doubt set your main weapon to every slot, relog, then re-set the quick-slots.

More of Gorgon's explanation:

Now the real weird part is why. Quick-slots can track their target items by internal server object id until you log out, but also use inventory position - specific equipped positions, location in main inventory, or the location of a container + position in that container. Usually that is fine because that should always be correct. If you move anything in inventory, the quick-slot gets updated at the same time, so both are saved.

If the item was in a bag, and the bag was dropped/removed before the save (more on that later), then your quick-slot is going to look in the wrong place when you re-log. Bioware planned for this somewhat by adding a check during log-in that will clear the slot if nothing is found at the location, but things get messy when something else is there. If it is something that can be quick-slotted, it will be, and you see the new icon even. In your case, it was a key and couldn't be quick-slotted, but it isn't smart enough to empty the slot info. Since it can't use the key, the icon isn't used either, so it looks blank too.

Stuff in containers is even more silly. Quick-slots clear themselves when you drop the item out of normal inventory, but If you quick-slot an item in a bag (or place an item in one after slotting), the slot isn't cleared when you drop the bag. If you place another bag in the same inventory location, with an item in the same bag spot, then re-log, it will try to slot the new item.

Actually, after reading thru that a few time I think I finally understand what happened:Fighting giant-kin with a DM'd group in Deglos, and for fun I was using an exotic weapon, which needed a feat from an item of clothing. Server crashed me out, on relog the game engine checks .bic for exotic feat, not finding it (because it's on gear that it doesn't check at this stage) it unequipped the weapon and dumped into the first space of my inventory... the rest of the inventory is then shuffled around that item - re-arranging various bags in an irritatingly random fashion. Now I know I had a few gadgets in bags that were quick-slotted, so I'm guessing the re-arranging dumped my bag with the keys into the location that my gadget bag used to be, and then as per Gorgon's explanation above, it tried to quick-slot the item in the same location in that bag, which unfortunately was a key.

After near a year of constant error messages from this poor toon, finally a super-simple fix!

Thanks to everyone who had a look over this, and hopefully this will prove useful to someone in the future!